This invention relates to an image forming apparatus which forms images by an electrophotographic method, and in particular, to an image forming apparatus which forms double-sided copies (hereinafter referred to as a duplex image forming for simplicity's sake) by forming toner images on both sides of a transfer material and fixing them together at time.
Duplex copying, wherein toner images are formed on both sides of the transfer material, has been known in the prior art. In such methods, the image for one side is transferred onto the transfer material and fixed; thereafter, the transfer material is reversed by manipulation within the machine and the second image is transferred onto the second side and is thereafter fixed.
In this type of duplex copying apparatus, as mentioned above, the transport of the transfer material is carried out in such a manner as to feed it to the reverse paper feeding device for duplex and to make it go through the fixing apparatus twice; hence, a long processing time for copying is required due to the long transport path of the transfer material, and further, it decreases the reliability of transfer material transport and causes a paper jam to occur frequently that the transfer paper, which is made curled due to having once passed through the fixing apparatus, is again transported.
On the other hand, the applicant has been studying a method of duplex image forming wherein a toner image formed on an image bearing member (a first image bearing means) is temporarily transferred onto an intermediate transfer member (a second image bearing means). Then, the front toner image is formed on the image bearing member and the transfer material is introduced between the image bearing member and the intermediate transfer member. With proper timing, the reverse toner image is transferred from the intermediate transfer member to the reverse side of the transfer material and the front toner image is transferred directly from the image bearing member to the front side of the transfer material. These transfers are carried out simultaneously.
Such a method of duplex image forming as mentioned above, can solve the above-described problem but has had a problem too. In the duplex image forming apparatus employing such a method, there is provided a stored program for image forming compiled in advance, and at the time of duplex image forming, the following are controlled in accordance with the stored program: the timing of driving for rotation of the image bearing member and the intermediate transfer member, the timing of image writing by charging and image exposure, and the timing of the feeding of the transfer material to the transfer area, and a front side image and a rear side image are formed on the both sides of a transfer member; heretofore, studies have been carried out on the above-mentioned method.
However, because an image bearing belt, which will be explained later, is used for the intermediate transfer member, it is difficult to prevent a speed fluctuation due to the elongation (the variation of the peripheral length) of the intermediate transfer member in use, the slippage between the driving roller and the intermediate transfer member, the load variation to the drive motor for driving the intermediate transfer member, etc. Because of the foregoing fluctuations, it has been found that copies are made wherein the leading edge of the front toner image and/or the leading edge of the rear toner image do not coincide with the leading edge of the transfer material.